Solution-Processable Cu(II) Phthalocyanine Derivative as Dopant

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Solution-Processable Cu(II) Phthalocyanine Derivative as Dopant-Free Hole Transport Layer for Efficient and LowCost Rutile TiO2 Array-Based Perovskite Solar Cells Shufang Wu, Chi Chen, Qingwei Liu, Tianyou Peng, Renjie Li, and Jing Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01115 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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ACS Applied Energy Materials

Solution-Processable Cu(II) Phthalocyanine Derivative as Dopant-Free Hole Transport Layer for Efficient and LowCost Rutile TiO2 Array-Based Perovskite Solar Cells Shufang Wu, Chi Chen, Qingwei Liu, Tianyou Peng,* Renjie Li,* and Jing Zhang College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China.

ABSTRACT: The full-scale commercialization of the organometallic perovskite solar cells, as one of the most high-efficiency photovoltaic technology, needs to lower the cost and improve the long-term stability by further optimizing the device architecture and component materials. In this context, a solution-processable tert-butyl substituted Cu(II) phthalocyanine (CuPc(tBu)4) is employed as dopant-free hole transport layer to fabricate a low-cost and high-efficiency perovskite solar cell with a low-temperature processed rutile TiO2 array as electron transport layer and a mixed-ion perovskite (Cs0.05(FA0.83MA0.17)0.95Pb(I0.9Br0.1)3) as light absorber. It is found that the optimized rutile TiO2 array film-based solar cell with the dopant-free Cu(II) phthalocyanine hole transport layer achieves a power conversion efficiency of 13.7% with less hysteresis, which can be further enhanced to 14.8% after introducing Al2O3 buffer layer between the Cu(II) phthalocyanine hole transport layer and the perovskite layer due to the improvement of the interfacial contact. These results boost up the potential of solution-processable Cu(II) phthalocyanine derivatives as low-cost and stable hole transport layers for high-efficiency perovskite solar cells, and thus provide an important future direction for the full-scale commercialization of perovskite solar cells. KEYWORDS: perovskite solar cell, Cu(II) phthalocyanine, dopant-free hole transport material, buffer layer, power conversion efficiency

1. INTRODUCTION Organometallic perovskite materials have become the research hotspots in the fields of photovoltaic technology due to the huge success of their applications in the perovskite solar cells (PSCs), which are thought to be a promising alternative to the commercial Si-based photovoltaic devices since the power conversion efficiency (PCE) has achieved an unprecedented progress from the first 3.8% to the newly recorded 22.1% in the past few years.1-4 However, there are still some obstacles to its commercialization process. For instance, most highefficiency PSCs are based on the conventional device architecture, in which the mesoporous TiO2 film as electron transport layer (ETL) needs to be fabricated through high-temperature process, and the 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) or polytriarylamine (PTAA) as hole transport layer (HTL) has some limitations such as very expensive, highly difficult preparation, and serious stability problems.5,6 To break through the above difficulties, many efforts have been made to improve the PCE, stability and cost performance of PSCs through fabrication process modification, interface engineering, device configuration and component design.7-21 For example, planar heterojunction architecture or lowtemperature processed ETLs have been proposed to fabricate PSCs so as to avoid the time-/energy-consuming hightemperature process for the fabrication of mesoporous TiO2based ETLs.12-14 Also, hole transport material (HTM)-free PSCs using carbon counter electrode are proposed to reduce

the fabrication cost and improve the long-term stability, and the PCE of this kind of PSCs has been boosted to more than 14.0%.15-17 In addition, more and more efforts have focused on the development of low-cost and thermally stable HTMs so as to replace the very expensive spiro-OMeTAD or PTAA that needs the usage of hygroscopic p-type dopants, which usually cause the deterioration of the device performance during a long-term operation period.18-21 Among various explored HTMs, metal phthalocyanines (MPcs, M = Cu, Zn, Ni or Pd) with excellent chemical and thermal stability are the potential alternative HTMs of PSCs since the strong π-π interactions between MPc molecules bearing 18 conjugated π electrons usually result in specific molecular ordering, high crystalline and robust stack with high hole mobility.22-34 For instance, some MPcs such as HT-ZnPc and TB-CuPc film exhibited a conductivity of 8.0 × 10−5 and 4.0 × 10−5 S cm−1,27 respectively. It is comparable with that (4.7 × 10−4 S cm−1) of spiroOMeTAD,35 suggesting that an optimal packing of MPc within thin films would lead to proper photovoltaic output of PSCs.35 At the early stage, most MPc HTLs were deposited through vacuum evaporation process.18-20 Since it is hard to be popularized owing to the strict requirement of equipment, solution-processing deposition becomes a more favorable choice because it is easy to implement and control by using simple equipment.26-34 For example, Nazeeruddin’s group have synthesized a series of solution-processable ZnPcs bearing various peripheral substituents, and found that the peripheral substituents can not only modify the light absorption property and energy band structures, but also affect the molecular ag-

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gregation behavior and charge transport ability of ZnPcs,27-29 and a PCE of 17.2% measured under reverse scanning mode was obtained when using tetra-5-hexylthiophene substituted ZnPc as HTM and a mixed-ion perovskite as light absorber.27 Also, the PSC with a CuPc derivative as HTM exhibited a high PCE of ∼18% and excellent thermal stability (maintained 97% of its initial efficiency after 1000 h of thermal annealing at 85 °C).30 Nevertheless, most MPc derivatives as HTL still need usage of additives such as bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) and/or 4-tert-butylpyridine (TBP), which would threaten the long-term stability of devices. Sun’s group has reported that the champion PSC based on dopant-free NiPc-(OBu)8 layer as HTL only showed an average PCE of 9.9%, while the PSC containing NiPc-(OBu)8 HTL doped with LiTFSI and TBP exhibited much higher average PCE of 17.0%.26 Alternatively, a “p-type” metal oxide (V2O5) buffer layer was introduced to the dopant-free NiPc-(OBu)8 HTL-based PSC, which displayed an impressive average PCE of 17.6%.26 This is a record PCE value for solutionprocessable dopant-free MPc HTL-based PSCs to the best of our knowledge. Recently, we have fabricated vertically oriented nanoneedles/nanosheets rutile TiO2 array (RTA) film through a lowtemperature (70 oC) chemical bath deposition (CBD) process, and utilized it as ETL of a spiro-OMeTAD HTL-based PSC with a PCE of 15.4%, which was slightly improved to 16.3% after aging and storage in the dark.34 Herein, an efficient and low-cost PSC consisting of the low-temperature processed RTA film as ETL, a solution processable tert-butyl substituted CuPc (CuPc(tBu)4) as dopant-free HTL, and a mixed-ion perovskite (Cs0.05(FA0.83MA0.17)0.95Pb(I0.9Br0.1)3) as light absorber was successfully fabricated. After optimizing the CuPc(tBu)4 concentration, the corresponding dopant-free CuPc(tBu)4 HTL-based PSC achieves a PCE of 13.7%, very similar to that (13.9%) of the dopant-free spiro-OMeTAD HTL-based device even though the PCE of the doped spiro-OMeTAD HTL-based one can be significantly improved to 18.0%. After introducing Al2O3 buffer layer to engineer the aggregation of CuPc(tBu)4 molecules and act as an additional blocking layer, the dopantfree CuPc(tBu)4 HTL-based PSC displays an improved PCE of 14.8%. The functions of the dopant-free CuPc(tBu)4 HTL on the perovskite/RTA film were intensively explored based on the results of spectroscopic, electrochemical and photoelectrochemical measurements of the corresponding devices.

2. EXPERIMENTAL SECTION 2.1. Material Syntheses. Lead iodide (PbI2), lead bromide (PbBr2) and formamidinium iodide (FAI) were purchased from Wuhan Jinge Solar Energy Tech Co. Ltd. 2,2',7,7'tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) was purchased from Shenzhen Feiming Sci & Tech Co. Ltd. Cesium iodide (CsI) was bought from Aladdin. Bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) was bought from J & K Chemical. 4-tert-butylpyridine (TBP) was purchased from Sigma-Aldrich. All other chemicals were purchased from Sinopharm Chemical Reagent Corporation and used as received unless noted otherwise. Methylammonium iodide (MAI) and methylammonium bromide (MABr) were synthesized according to the previously reported procedure.23 The precursor solution of mixed-ion perovskite (Cs0.05(FA0.83MA0.17)0.95Pb(I0.9Br0.1)3) was prepared by dissolved FAI (1.0 M), PbI2 (1.1 M), MABr (0.2 M), PbBr2

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(0.2 M) and CsI (0.075 M) in an anhydrous dimethylformamide (DMF)/dimethyl sulfoxide (DMSO) solution with a volume ratio of 4:1, which was filtered through 0.22 µm PVDF filter before the film deposition process. CuPc(tBu)4 was prepared in our laboratory according to a one-step synthetic route as shown in Figure S1, which is similar to our previous report.23 TOF-MS (m/z) calcd. for C48H48N8Cu [M+H]+ 800.5, found 798.8 (Figure S2). 2.2. Rutile TiO2 Array Preparation. FTO substrate was sequentially cleaned by detergent, water, acetone, 2-propanol and ethanol in an ultrasonic bath and dried by air. The rutile TiO2 array (RTA) film on FTO glass substrate was prepared via a CBD process according to our previous report.34 Typically, the cleaned FTO glass with conductive coating facing down was immersed into the reaction solution containing 20 mL of water, 0.3 mL of HCl solution (37%), and 0.3 mL of TiCl3 solution (15% in HCl solution) in a glass bottle. The CBD process was performed at 70 oC for 60 min in a laboratory oven. After that, the resultant RTA film was taken out and rinsed with water and ethanol several times, and then dried in air and annealed at 100 oC for 1 h. 2.3. Perovskite Solar Cell Fabrication. Firstly, the above RTA film was treated with O3/ultraviolet for 15 min. The preparation of perovskite layer was conducted in an Ar-filled glovebox through a solvent engineering deposition method. Typically, the perovskite precursor solution was spin-coated at 1000 rpm for 10 s and then 5000 rpm for 20 s onto the RTA film, and chlorobenzene (100 µL) as anti-solvent was dropped 10 s prior to the end of the second step. The perovskite layer was further annealed at 100 °C for 40 min to remove the solvent. After cooling down to room temperature, CuPc(tBu)4 dissolved in chlorobenzene with a concentration range of 0-25 mg mL-1 was spin-coated on the top of perovskite layer. For comparison, spiro-OMeTAD was deposited as reference by using its chlorobenzene solution (72.3 mg mL-1) containing 28.8 µL of TBP and 17.5 µL of LiTFSI acetonitrile solution (520 mg mL-1) as additives. Finally, a thin gold electrode was thermally evaporated on the HTM layer under high vacuum. 2.4. Material Characterization and Photoelectrochemical Test. X-ray diffraction (XRD) patterns were recorded using a Miniflex 600 X-ray diffractometer with Cu Kα irradiation (λ = 0.15418 nm) at 40 kV and 15 mA and a scan rate of 4o min-1 in the range of 10o ≤ 2θ ≤ 60o. The surface and crosssection morphologies of the films were investigated by a field emission scanning electron microscope (FESEM, Zeiss Sigma). UV-Vis diffuse reflectance absorption spectra (DRS) were obtained by using a Shimadzu UV-3600 spectrophotometer. Photoluminescence (PL) spectra were determined by a K2 ISIS spectrometer. Time-resolved photoluminescence (TRPL) spectra were conducted on FES 920 fluorescence spectrophotometer (Edinburgh Instruments) with excitation wavelength of 377 nm and detection wavelength of 780 nm. MALDI-TOF-MS spectra were taken on a Bruker BIFLEX III ultrahigh resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer with α-cyano-4-hydroxycinnamic acid as a matrix. Electrochemical measurements were carried out with a BAS CV-50 W voltammetric analyzer as described in our previous report.23,36 The device was illuminated by a 300 W solar simulator (Newport, 91160) with a power density of 100 mW cm-2 (AM1.5G). The active area is 0.09 cm-2, which was defined by a shadow mask. The light intensity was determined using a reference monocrystalline silicon cell (Oriel). A computer-


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controlled Keithley 2400 sourcemeter was employed to collect the photocurrent-voltage (J-V) curves of the PSCs. The incident photon-to-electron conversion efficiency (IPCE) was measured by using a QE-R 3011 system (Enli Technology Co. Ltd. China) in the wavelength range of 300-850 nm. For the open-circuit voltage decay (OCVD) curve measurement, the illumination was turned off using a shutter after the device was first illuminated to a steady voltage, and then the OCVD curve was recorded. The above measurements were carried out on a CHI 604C electrochemical analyzer.

3. RESULTS AND DISCUSSION 3.1. Device Configuration and Microstructure Analyses. The device configuration of the fabricated PSC with CuPc(tBu)4 as dopant-free HTL is depicted in Figure 1a, in which a vertically oriented nanoneedles/nanosheets rutile TiO2 array (RTA) film (Figure S3a) synthesized via the lowtemperature CBD process was employed to replace the traditional mesoporous TiO2-based ETL,34 and the perovskite (Cs0.05(FA0.83MA0.17)0.95Pb(I0.9Br0.1)3) layer was fully covered on the RTA film surface (Figure S3b). The crystal phase and purity of the perovskite layer can be estimated through the XRD patterns (Figure S4), in which the main diffraction peaks can be ascribed to the photoactive trigonal α-phase (black phase) without any signal of photoinactive δ-phase (yellow phase).37 This high-purity black phase of perovskite can ensure an excellent photoelectrochemical property even though a few PbI2 residues might exist in the perovskite layer as its characteristic diffraction peak showed at 2θ = 12.7°. Moreover, no obvious difference is observed from the XRD patterns of perovskite/RTA and CuPc(tBu)4/perovskite/ RTA films (Figure S4), implying that the disposition process of CuPc(tBu)4 HTL does not influence the perovskite property on the RTA film.

Figure 1. Schematic view and energy band diagram (a), crosssectional FESEM image (b) of the RTA film-based PSC with CuPc(tBu)4 as HTL.

According to the previous literatures,23,31,33 the optical bandgap (E0-0 = 1240/λint.) of CuPc(tBu)4 can be estimated to be ∼1.70 eV from the intersection point (λint. = ∼729 nm) of the normalized UV-vis absorption and photoluminescence (PL) spectra (Figure S5). Since the first half-wave redox potential (Eox vs. SCE) that can be obtained from the cyclic voltammogram (CV) curve of CuPc(tBu)4 (Figure S6) corresponds to the highest occupied molecular orbital level (EHOMO),36,38 the EHOMO of CuPc(tBu)4 can be calculated to be -5.20 eV based on the formulae (EHOMO = −(Eox + 4.71) eV) as shown in Table S1, and thus the lowest unoccupied molecular orbital level (ELUMO) is estimated to be −3.50 eV by using the equation (ELUMO = EHOMO + E0–0),36,38 where E0–0 is 1.70 eV as mentioned above. Moreover, it was reported that the mixed-ion perovskite has valence band (VB) and conduction band (CB) of −5.65 eV and −4.05 eV,31,39 respectively. Based on the above results and discussion, the schematics of band alignment for the dopant-free CuPc(tBu)4 HTL-based PSC can be shown in Figure 1a. As seen, the EHOMO (-5.20 eV) of CuPc(tBu)4 is higher than the VB (-5.65 eV) of perovskite, indicating that the photoinduced hole transport from perovskite layer to CuPc(tBu)4 HTL via the intimate interfacial contact is allowed thermodynamically.23,26,30 In addition, the ELUMO (-3.50 eV) of CuPc(tBu)4 is higher than the CB (-4.05 eV) of perovskite, which could block the flow of photoinduced electrons from perovskite layer to CuPc(tBu)4 HTL, and thus benefits the directional transport of the photoinduced electrons in the PSC.23 Figure 1b reveals the RTA film-based PSC with CuPc(tBu)4 as dopant-free HTL has a well-defined layer-bylayer structure with clear interfaces, and the perovskite capping layer with a thickness of ∼400 nm is uniformly covered on the RTA film (also ref. Figure S3b), which facilitates the harvest of incident photons and prevents the penetration of HTM into the RTA film (ETL). Although the thickness of CuPc(tBu)4 layer is hard to determine from this cross-sectional EFSEM image, it is believed that the CuPc(tBu)4 layer can reduce the shunt paths between perovskite film and Au electrode.40 3.2. Effects of CuPc(tBu)4 Concentration on Device Performance. It is well known that the HTM concentration has a great influence on the photovoltaic performance of PSCs.23,24 As for the present CuPc(tBu)4 HTM, its solution concentration also determines the thickness and homogeneity of the spin-coated HTL, and then influence the photoinduced charge extraction. The CuPc(tBu)4 HTL spin-coated with 10 mg mL-1 CuPc(tBu)4 solution has a smooth surface (Figure 2a), while a 20 mg mL-1 CuPc(tBu)4 solution leads to rougher surface co-existed with obvious CuPc(tBu)4 agglomerations (marked with red circles in Figure 2b). These agglomerations not only are disadvantage to form benign contacts at perovskite/CuPc(tBu)4/Au interfaces, but also bring down the orientation of CuPc(tBu)4 molecules and even act as recombination centers.40 All of them would be detrimental to the hole transport ability of CuPc(tBu)4 HTL, and then causing a decreased conversion efficiency of PSCs. To optimize the CuPc(tBu)4 concentration, various CuPc(tBu)4 HTL-based PSCs were fabricated by spin-coating CuPc(tBu)4 solutions with different concentrations. Figure 3a depicts the typical photocurrent density-voltage (J-V) curves of those PSCs measured under simulated AM 1.5G solar irradiation (100 mW cm-2) with a forward scanning mode, and the corresponding photovoltaic performance parameters are listed



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in Table 1. As is well known, perovskite materials can transport electrons and holes due to their ambipolar and longrange diffusion features,41 and thus the PSC without CuPc(tBu)4 HTL still exhibits a PCE of 7.70% with a shortcircuit current density (Jsc) of 20.5 mA cm-2, an open-circuit voltage (Voc) of 0.65 V and a fill factor (FF) of 0.58 (Table 1). The relatively low Voc and PCE values mainly arise from the serious interfacial charge recombination owing to the direct contact between perovskite film and Au electrode.42

Figure 3. J-V (a) and OCVD (b) curves of the PSCs with CuPc(tBu)4 as HTLs that were spin-coated using CuPc(tBu)4 solutions with different concentrations.

Figure 2. Top-view FESEM images of the CuPc(tBu)4 HTLs spin-coated with 10 (a) and 20 (b) mg mL-1 CuPc(tBu)4 solution on the perovskite/RTA film.

After spin-coating 2.5 mg mL-1 CuPc(tBu)4 solution on the perovskite/RTA film, the PCE does not change much possibly due to the low CuPc(tBu)4 concentration, which is not enough to form CuPc(tBu)4 HTL fully covering the perovskite layer, and thus cannot effectively extract/transport the photoinduced holes. Nevertheless, the CuPc(tBu)4 HTL spin-coated with 5.0 mg mL-1 CuPc(tBu)4 solution can significantly improve the device performance (Table 1). Especially, the Voc value is 0.38 V higher than that (0.65 V) of the PSC without CuPc(tBu)4, and thus gains a much better PCE (13.1%). Upon enhancing the CuPc(tBu)4 concentration to 10 mg mL-1, a PCE of 13.7% with enhanced Jsc (22.1 mA cm-2), Voc (1.04 V) and FF (0.60) can be achieved. After that, the performance parameters of PSCs show decreasing trends along with further enhancing the CuPc(tBu)4 concentration from 10 to 25 mg mL-1 (Table 1). Namely, the CuPc(tBu)4 HTL-based PSC fabricated by spincoating 10 mg mL-1 CuPc(tBu)4 solution achieves the best photovoltaic performance with a PCE up to 13.7%. Since the spectral absorption ability of the perovskite/RTA film does not change much after spin-coating the CuPc(tBu)4 HTL (Figure S7), which may be ascribed to its very thin thickness. Therefore, it can be conjectured that the obviously improved photovoltaic performance along with enhancing CuPc(tBu)4 concentration from 0 to 10 mg mL-1 mainly arises from the more effective hole transport from perovskite layer to Au electrode and the blocking effect of electron leakage due to

Table 1. Photovoltaic Performance Parameters of the PSCs with Different HTLs Measured under Forward Scanning Mode HTL type without HTL CuPc(tBu)4 (2.5 mg mL-1) CuPc(tBu)4 (5.0 mg mL-1) CuPc(tBu)4 (10 mg mL-1) CuPc(tBu)4 (15 mg mL-1) CuPc(tBu)4 (20 mg mL-1) CuPc(tBu)4 (25 mg mL-1) spiro-OMeTAD doped spiro-OMeTADa

Jsc (mA cm-2) 20.5 22.9 21.9 22.1 20.0 20.6 17.4 22.6 23.2

Voc (mV) 0.65 0.72 1.03 1.04 1.04 1.05 1.04 1.03 1.11

FF 0.58 0.47 0.58 0.60 0.58 0.46 0.44 0.60 0.70

PCE (%) 7.70 7.80 13.1 13.7 12.0 10.0 8.0 13.9 18.0

a doped spiro-OMeTAD layer was deposited by using its chlorobenzene solution containing TBP and LiTFSI acetonitrile solution.

the formation of thicker CuPc(tBu)4 HTL. Once the CuPc(tBu)4 concentration is too higher, however, a rougher CuPc(tBu)4 HTL with reunited particles on the surface would be generated (Figure 2b), which not only increases the series resistance of the PSC, but also reduces the interfacial contact and the π-π stacking of CuPc(tBu)4 molecules, and thus causing the decreased Jsc, FF and PCE values of the devices (Table 1).40 In addition, it was reported that too thicker CuPc(tBu)4 film could absorb more photons that pass through the perovskite layer and reflected by the Au electrode, which reduce the available light for the perovskite layer, and then causing the decline of Jsc value since the absorbed photons of CuPc(tBu)4 HTL do not contribute to photocurrent.42 As a consequence, the device fabricated by spin-coating 25 mg mL-1 CuPc(tBu)4 only exhibits a PCE of 8.0% with much lower Jsc and FF val-


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ues. The above results demonstrate that the CuPc(tBu)4 layer spin-coated at a relatively low concentration would be a costeffective dopant-free HTL for PSCs. 3.3. Device Photoelectrochemical Behavior Analyses. The open-circuit voltage decay (OCVD) curves of various CuPc(tBu)4 HTL-based PSCs fabricated by spin-coating CuPc(tBu)4 solutions with different concentrations were performed to understand the charge recombination processes.23 As shown in Figure 3b, the device fabricated by spin-coating 2.5 mg mL-1 CuPc(tBu)4 solution exhibits the fastest decay trend of Voc under dark, which maintains only ~0.2 V of voltage after 80 s decay, while the devices fabricated by spincoating 25 and 10 mg mL-1 CuPc(tBu)4 solutions can keep remained voltage of ~0.3 and ∼0.5 V, respectively. It indicates that the CuPc(tBu)4 HTL spin-coated with 10 mg mL-1 CuPc(tBu)4 solution has more effective hole transport from perovskite layer to Au electrode and the blocking effect of electron leakage in the device. As mentioned above, too thin CuPc(tBu)4 HTL does not form an energy barrier to extract holes from the perovskite layer and block the shunt paths, while too thick one with reunited particles is detrimental to the interfacial contact and hole transport ability, which then lead to the aggravation of charge recombination and the reduction of device performance.

recombination at interface,16,34,39,43 and then resulting in improved device performance, which is consistent with our J–V data in Figure 3a and Table 1. PSCs often exhibit anomalous photocurrent hysteresis, which would reduce the reliability of efficiency measurements.44 To estimate the impact of hysteresis on the performance of our PSCs, the J-V curves of CuPc(tBu)4 HTL-based PSCs fabricated by spin-coating 10 mg mL-1 CuPc(tBu)4 solution were measured under forward and reverse voltage scanning modes and displayed in Figure 4. The device showed a negligible hysteresis behavior and very similar PCE values under the opposite measurement directions. It indicates that the CuPc(tBu)4 HTL can effectively extract holes and avoid the charge accumulation at interface, which then eliminate the hysteresis and improve the reliability of device efficiency. The reproducibility of CuPc(tBu)4 HTL-based PSCs was verified by fabricating and testing 15 cells, and the corresponding performance parameters as listed in Table S2. The PCEs are mainly distributed in the range of 11.9~13.5% with an average value of 12.7%, implying the relatively good reproducibility of the dopant-free CuPc(tBu)4 HTL-based PSC.

Figure 4. Effects of the voltage scanning mode on the photovoltaic performance of the PSC with CuPc(tBu)4 as HTL that was spincoated by using 10 mg mL-1 CuPc(tBu)4 solution.

To further investigate the effect of CuPc(tBu)4 concentration on the charge transport dynamic process in the devices, time-resolved photoluminescence (TRPL) spectra were measured and depicted in Figure S8. Those TRPL spectra of devices fabricated with different CuPc(tBu)4 concentrations exhibit an average PL lifetime order of 2.5 mg mL-1 (15.2 ns) > 25 mg mL-1 (14.6 ns) > 10 mg mL-1 (8.7 ns). The obviously increasing PL quenching rate of the PSC fabricated with 10 mg mL-1 CuPc(tBu)4 solution can be due to the enhancing interfacial contacts of Au/CuPc(tBu)4/perovskite film as mentioned above, which then result in an improved charge carrier extraction ability of PSC.34,43 Whereas the PL lifetime (14.6 ns) of PSC fabricated with 25 mg mL-1 CuPc(tBu)4 solution is longer than that (8.7 ns) of the device fabricated with 10 mg mL-1 CuPc(tBu)4 solution, suggesting that the rougher CuPc(tBu)4 HTL with reunited particles could be detrimental to the interfacial contacts and hole transport ability. Namely, the fastest PL quenching rate of PSC fabricated with 10 mg mL-1 CuPc(tBu)4 solution among those devices tested demonstrates its more efficient charge extraction and suppressed charge

Figure 5. J-V curves (a) and steady-state photocurrent density and efficiency held at a forward applied bias of the maximum output power point (b) of the PSCs with different HTLs.

For comparison, the PSCs based on spiro-OMeTAD HTLs without/with dopants were also fabricated under the same conditions. From the J-V curves (Figure 5a) and the corresponding photovoltaic data (Table 1), it can be found that the PCE (13.7%) of the dopant-free CuPc(tBu)4 HTL-based PSC fabricated by spin-coating 10 mg mL-1 CuPc(tBu)4 solution is comparable to that (13.9%) of the dopant-free spiro-OMeTAD HTL-based one. After using TBP and LiTFSI as dopants, the corresponding spiro-OMeTAD HTL-based PSC exhibits a much higher PCE (18.0%) with enhanced Jsc, Voc and FF val-



ues (Table 1), which can be due to the improved conductivity of the doped spiro-OMeTAD HTL.26 Also, the steady-state current density and PCE held at a forward bias of the maximum point indicate that the dopant-free CuPc(tBu)4 HTLbased PSC exhibits a stable photocurrent density of ~15.9 mA cm-2 and PCE of 12.4% under 0.78 V (Figure 5b), which are slightly lower than that of the doped spiro-OMeTAD HTLbased one. These demonstrate that the dopant-free CuPc(tBu)4 HTL with low-cost, superior chemical and thermal stability would have potential in the future commercialization of PSCs. 3.4. Effects of buffer layer on the device performance. It was reported that the introduction of buffer layer (such as Al2O3, SiO2, ZrO2) between perovskite layer and HTL could be an effectual strategy to improve the device performance.40,45,46 Generally, a buffer layer usually plays two main roles: 1) as a porous framework to ameliorate the uniformity of HTL and the interfacial contact; 2) as a blocking layer to reduce the current leakage paths between perovskite layer and Au electrode.45 Therefore, a thin Al2O3 buffer layer was deposited on the perovskite film by spin-coating 3 mg mL-1 Al2O3 isopropanol solution for further improving the photovoltaic performance of CuPc(tBu)4 HTL-based PSCs. Figure S9 shows the top-view and cross-sectional EFSEM images of the Al2O3 layer and CuPc(tBu)4/Al2O3 layer on the perovskite films. Although the deposition of Al2O3 makes no obviously change in the surface morphology of the perovskite film due to the very thin thickness and tiny Al2O3 particles (Figure S3b, S9a and b), the Al2O3 layer deposited on the perovskite film could lead to the formation of more uniform and smoother CuPc(tBu)4 HTL without reunited particles even though 20 mg mL-1 CuPc(tBu)4 solution was spin-coated (Figure 2b, S9c and d). It indicates that the buffer layer can alleviate the aggregation of CuPc(tBu)4 molecules at higher concentration and improve the uniformity and orientation of CuPc(tBu)4 HTL.

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The effects of Al2O3 buffer layer on the performance of the CuPc(tBu)4 HTL-based PSCs fabricated by spin-coating CuPc(tBu)4 solutions with different concentrations were estimated. From the J-V curves and photovoltaic performance data listed in Figure 6 and Table 2, it can be found that the Al2O3 buffer layer improves the device performance in varying degrees. Among them, the highest PCE was still achieved by the CuPc(tBu)4 HTL-based PSC fabricated by spin-coating 10 mg mL-1 CuPc(tBu)4 solution, which is 14.8% with Jsc of 23.4 mA cm-2, Voc of 1.05 V and FF of 0.60. Moreover, the enhancement ratios of PCE are enlarged with enhancing the CuPc(tBu)4 concentration (Table 2). It means that the Al2O3 buffer layer has less effect on the dopant-free CuPc(tBu)4 HTL-based PSCs fabricated with low CuPc(tBu)4 concentrations, and mainly acts as a blocking layer under this condition due to the easy formation of orderly stacking and smooth CuPc(tBu)4 HTL. As for the devices fabricated with high CuPc(tBu)4 concentrations, the Al2O3 buffer layer also acts as a skeleton to regulate the aggregation of CuPc(tBu)4 molecules to improve the quality of CuPc(tBu)4 HTL, and then promoting charge separation/transport with reduced recombination. As a result, more significant improvement is observed for these PSCs fabricated with high CuPc(tBu)4 concentrations. For instance, the Al2O3 buffer layer in the CuPc(tBu)4 HTL-based PSC fabricated with 25 mg mL-1 CuPc(tBu)4 solution leads to the PCE enhancing from 8.0% to 11.9% with an improvement of 48.8% (Table 2). These results demonstrate the utilization of buffer layer is a good choice during the optimization of dopant-free CuPc(tBu)4 HTL.

Figure 6. J-V curves of the PSCs with Al2O3 buffer layer and CuPc(tBu)4 HTLs that was spin-coated using CuPc(tBu)4 solutions with different concentrations.

Table 2. Photovoltaic Performance of PSCs with Al2O3 Buffer Layer and CuPc(tBu)4 HTLs Derived from SpinCoating Different CuPc(tBu)4 Concentrations CuPc(tBu)4 5.0 mg mL-1 10 mg mL-1 15 mg mL-1 20 mg mL-1 25 mg mL-1

Jsc (mA cm-2) 22.9 23.4 20.9 21.3 19.8

Voc (mV) 1.05 1.05 1.06 1.06 1.07

FF 0.56 0.60 0.61 0.54 0.56

PCE (%) 13.4 14.8 13.5 12.1 11.9

increase ratio (%) 2.3 8.0 12.5 21.0 48.8

Figure 7. Effects of the Al2O3 buffer layer on the dark J-V (a) and IPCE (b) curves of the PSCs with CuPc(tBu)4 as HTLs that was spin-coated using 10 mg mL-1 CuPc(tBu)4 solution.


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To further investigate the effects of Al2O3 buffer layer on the performance of CuPc(tBu)4 HTL-based PSC fabricated with 10 mg mL-1 CuPc(tBu)4 solution, the dark J-V curves are measured. As can be seen from Figure 7a, the Al2O3 buffer layer can reduce the dark current stemming from the decrease of charge recombination, which is a benefit to gain a higher Voc value.43 Also, the Al2O3 buffer layer results in higher IPCE values in the whole wavelengths measured (Figure 7b), consistent with the enhanced Jsc values of the device (Table 1 and 2). The above results reveal that the Al2O3 buffer layer can facilitate the photovoltaic conversion due to more effectively charge extraction. The above conjecture can be validated by the OCVD curves (Figure 8a). The Voc value of the PSC without Al2O3 buffer layer drops to ~0.42 V after 80 s decay, while that of the PSC with Al2O3 buffer layer keeps ~0.50 V voltage, implying that the later has a slower Voc decay rate. In addition, the PSC with Al2O3 buffer layer exhibits a longer electron lifetime than the device without Al2O3 as shown in the corresponding electrons lifetime (τn)-Voc curves (Figure 8b). These results indicate again the Al2O3 buffer layer can reduce the charge recombination due to its dual functions as mentioned above.45 Namely, the Al2O3 buffer layer could play as a framework to enhance the uniformity and orientation of CuPc(tBu)4 HTL and as barrier to block the shunt paths. Both of them can improve the interfacial contact and inhibit the formation of leakage channels, which then facilitate the charge extraction and reduce the recombination, and thus causing the improved performance of the dopant-free CuPc(tBu)4 HTL-based PSCs.

Finally, the long-term stability tests for the PSCs fabricated with dopant-free CuPc(tBu)4 and doped spiro-OMeTAD as HTM layer, which were stored without encapsulation at ambient condition with a humidity of ∼30% in the dark, were conducted and depicted in Figure S10. As can be seen, the dopantfree CuPc(tBu)4 HTL-based PSC with Al2O3 buffer layer showed an excellent long-term stability and presented a small deterioration with ∼90% of its initial PCE maintaining after the aging time (336 h), while the doped spiro-OMeTAD-based device only kept 76% of its initial PCE. These results demonstrate that the CuPc(tBu)4 HTL-based PSC has obviously improved long-term stability as compared to the spiro-OMeTAD HTL-based device. It was reported that those dopants in the spiro-OMeTAD-based device are hygroscopic and water could accelerate the decomposition of the perovskite film, which might be the main reason for the rapid decline in PCE of the doped spiro-OMeTAD-based one.47,48 Whereas the hydrophobic characteristic of CuPc(tBu)4 can effectively prevent moisture penetration into the perovskite layer, thus resulting in an excellent long-term durability of the present dopant-free CuPc(tBu)4 HTL-based PSC under ambient conditions.31 The results presented here demonstrate that solution-processable CuPc derivatives have a great promise as an alternative HTMs for exploring efficient and stable PSCs in the future.

4. CONCLUSIONS In summary, a low-cost and high-efficiency PSC was fabricated by employing a low-temperature processed rutile TiO2 array film as ETL and a solution-processable CuPc(tBu)4 as dopant-free HTL, in which the facile and low-temperature processed ETL can decrease the time and energy consumptions, and the dopant-free CuPc(tBu)4 HTL spin-coated with low concentration can reduce the cost and the risk of instability of PSCs. After optimizing the CuPc(tBu)4 concentration, the corresponding CuPc(tBu)4 HTL-based PSC achieves a PCE of 13.7%, comparable with that (13.9%) of the dopantfree spiro-OMeTAD HTL-based one. Moreover, introducing Al2O3 buffer layer between CuPc(tBu)4 HTL and perovskite layer can further improve the PCE to 14.8% since the buffer layer plays dual functions as framework to engineer the aggregation of CuPc(tBu)4 molecules and as blocking layer to prevent the shunt paths. Our results not only boost up the potential of solution-processable CuPc derivatives as low-cost and stable HTLs, but also demonstrate that the selection and optimization of the multi-layer components would be favor in the realization of low-cost, high-efficiency and stable PSCs for the full-scale commercialization.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acsami.xxxxxxx.

Figure 8. Effects of the Al2O3 buffer layer on the OCVD (a) and τn-Voc (b) curves of the PSCs with CuPc(tBu)4 as HTLs that was spin-coated using 10 mg mL-1 CuPc(tBu)4 solution.

Synthetic route, MALDI-TOF mass spectrum, CV curve, UV-vis absorption and PL spectra of CuPc(tBu)4; Top-view FESEM images of the RTA film and the perovskite/RTA film; X-ray diffraction (XRD) patterns for the RTA film and the perovskite/RTA film; UV-Vis diffuse reflectance absorption spectra (DRS) for the perovskite/RTA film and the CuPc(tBu)4/perovskite/RTA film; Top-view and cross-sectional FESEM images of the Al2O3 layer and CuPc(tBu)4/Al2O3 layer on the perovskite/RTA film; Stability tests for the PSCs fabricated with dopant-free CuPc(tBu)4 and



doped spiro-OMeTAD as HTM layer; Photovoltaic performance parameters of 15 PSCs fabricated with CuPc(tBu)4 as HTL (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (T. Y. Peng) *E-mail: [email protected] (R. J. Li). ORCID Tianyou Peng: 0000-0002-2527-7634 E-mail: [email protected]. Fax: +86-27-68752237.

Author Contributions The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21573166, 21271146, 20973128, 20871096), the Funds for Creative Research Groups of Hubei Province (2014CFA007), and the Natural Science Foundation of Jiangsu Province (BK20151247), China.

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Graphic entry for the Table of Contents (TOC).


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Solution-Processable Cu(II) Phthalocyanine Derivative as Dopant

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